1. Field of the Invention:
The present invention relates to a domestic electric water heater which is altered in a way such as to allow control of bacterial contamination, particularly the elimination of the Legionella pneumophila, while retaining a good power efficiency.
2. Description of the Prior Art
Since it was discovered, at the Bellevue Stratford Hotel in Philadelphia (1976), that the Legionella pneumophila, commonly called the legionellosis, could cause serious infections in humans, numerous studies have been made in order to understand better the agents having an effect upon the proliferation of this bacterium which is found, as it has since been discovered, particularly at the bottom of domestic electric water heaters. It is, on the other hand, known that the legionellosis, like many other bacteria, does not grow nor survive at temperatures above 46.degree. C.
The Applicant has conducted numerous studies in order to determine the parameters involved in the growth of the legionellosis bacterium inside domestic electric water heaters. Following the results obtained, the inventors have carried out works in order to perfect and test certain minor and low cost changes to be made to present-day water heaters which are most likely to reduce and practically eliminate bacterial contamination. These research works have, among others, shown that:
the temperature distribution in a conventional domestic water heater can explain the bacterium proliferation;
the bottom of the present-day water heater never exceeds 40.degree. C. even when there is no hot water consumption. This temperature corresponds to a zone of bacterial proliferation. A greater water consumption has the effect of holding the bottom of the water heater at a lower mean temperature which is nevertheless located within the zone of bacterial proliferation;
stratification of the temperature at the bottom of the tank is notable;
it is difficult to increase the temperature at the bottom of the tank when there is no water consumption;
by increasing the temperature at the bottom of the tank through a recirculation pump, the conditions favorable to the legionellosis do not exist. This solution is however costly and not very reliable;
considering the noted stratification phenomenon, the temperature at the bottom of the tank does not increase proportionately even when the temperature set on the thermostats of the heating elements is increased so that the use of a mixing valve at 70.degree. C. does not appear promising, based on the results of thermal fields. Besides, this has also been confirmed by bacterial studies;
insulating the bottom and changing the position of the thermostat do not appear to be the most promising techniques; and
by raising to 70.degree. C. the water temperature inside the tank (presently at 60.degree. C. in a conventional water heater) it is not possible to destroy the bacterium, the temperature at the bottom of the tank remaining below 60.degree. C.; by increasing it to 80.degree. C., there is practically no more legionellosis at the bottom of the tank.
The above works have shown that the legionellosis develops in the stagnant zone at the base of a conventional domestic water heater due to the stratification of the water, the water at the lowest temperature being found at the bottom of the tank. More specifically, the sheet of water located between the immersed lower heating element and the bottom of the tank is rather hot, being between 30.degree. C. and 50.degree. C. and, consequently is quite favourable to far the proliferation of pathological bacteria, particularly the legionellosis. It thus becomes obvious that the solution to the problem is to raise the water temperature in that zone. For this purpose, an attempt to solve the above problems includes the lowering of the inner lower heating element. This solution is however not recommended because it causes scaling which may, in the long run, damage the lower heating element.